Nowadays, the audio portion of TV signals is often broadcasted in both mono and stereo sounds; and multiplex or multilingual transmission method is also practised in a TV broadcasting system.
For the stereo broadcasting system, enhancement of both bass and treble bands tends to improve the sound quality; while, as for the multiplex system, the treble band is preferably attenuated to minimize the noise components carried by the high frequencies so that an optimum quality sound may hie achieved.
On the other hand, an audio signal reproduced from, e.g., a videotape recording has generally much more noises than a broadcast audio signal. Accordingly, i% is generally preferred to attenuate both bass and treble bands of frequencies for such reproduced audio signal.
In the art, varying levels of DC voltages have been commonly used to control the respective bass and treble bands of an audio signal. One of such known tone control systems is constructed to generate bass and treble control voltages which vary linearly with the operation of the bass and the treble adjust keys, respectively. The variable bass control voltage serves to adjust the bass frequency range of the audio signal to a desired level; and, similarly, the variable treble to control voltage adjusts the treble range of the audio signal to a predetermined level.
However, this type of conventional apparatus has the deficiencies of requiring a manual operation of the bass and the treble adjust keys, which are provided separately for different types of audio signals such as stereo, multiplex and reproduced audio signals.